1. Field of the Invention
The present invention relates generally to interlocking acetabular fixation screws and particularly relates to interlocking acetabular fixation screws for providing a solid fixation of an acetabular cup into a prepared acetabular cavity of a pelvis during a total hip arthroplasty. In a further and more specific aspect, the invention has to do with a combination of a new reverse hip acetabular cup having a stem firmly affixed in the concave portion thereof and the interlocking acetabular fixation screws which are used to firmly affix the cup in the acetabular cavity.
2. The Related Art
It can be appreciated that the securing of an acetabular cup with ordinary screws is known in the prior art. For example, acetabular cups are fixed using conventional cancellous screws with flat heads having a recess for insertion and extraction with common screwdrivers.
Other means of fixation include asperities and voids applied over the convex surface of the acetabular cup, which will allow bone ingrowth.
The main problem with conventional cancellous screws is the fact that they have the tendency to become loose by resorption of bone in the immediate proximity of the screws.
Another problem with conventional cancellous screws is micro motion caused by movement of the acetabular cup under dynamic load, which seems to initiate loosening and subsequently trigger osteolysis that will lead to bone resorption in and loosening of the implant.
Conventional cancellous acetabular screws also lack the rigidity of screw fixation to the cup under dynamic load, leading to micromotion at the point of contact between the acetabular cup and the screws. As a result, the micromotion leads to fretting and pitting. In addition, metallic debris and wear particulates may consequently be generated, causing local granulomas around the implant.
Interlocking bone screws have been used with conventional flat bone plates, and with acetabular cups such as described in published European Patent Application No. EP 1 800 626 A2. However, proper sizing of the screws has not previously been recognized as a factor in the viability of these screws because they have not been used with reverse hip acetabular cups.
Accordingly, existing cancellous bone screws have failed to adequately address the problem of providing a solid acetabular fixation means into a prepared acetabular cavity of a pelvis during a total hip arthroplasty.
In this respect, the interlocking acetabular fixation screws according to the present invention substantially depart from the conventional sizing of screws of the prior art, and in so doing provides an apparatus primarily developed for the purpose of providing a solid acetabular fixation means into the prepared acetabular cavity of the pelvis during a total hip arthroplasty, particularly in respect of securing the reverse hip acetabular cup of the invention.